now cells in the stomach known as the parietal and chief cells contribute to the formation of gastric juice parietal cells secrete two substances known as hydrochloric acid abbreviated hcl and a glycoprotein called intrinsic factor now hydrochloric acid contributes to the acidity low ph environment of the stomach intrinsic factor is required for the intestinal absorption of dietary vitamin b12 recall that the absence of intrinsic factor is the cause of pernicious anemia a blood disorder in which red blood cells fail to divide and grow in number as a result of deficient vitamin b12 now chief cells secrete only one substance the inactive enzyme precursor called pepsinogen hydrochloric acid converts inactive pepsinogen into active functional pepsin pepsin is the enzyme that chemically breaks down protein or large polypeptides into smaller polypeptides as a result we can establish that protein digestion first begins in the stomach because the site of release for gastric juice is the lumen or cavity of the stomach which is continuous with the body's external environment both chief cells and parietal cells are considered exocrine glands now recall that the stomach is also classified as a secondary endocrine gland another cell located in the stomach is the enteroendocrine cell which as the name implies is endocrine in function and therefore secretes hormones into the bloodstream specifically enteroendocrine cells secrete the hormone gastrin gastrin regulates numerous processes including stomach emptying this brings us to our next two checkpoint questions you will see two back to back in a series here is the first and here is the next now the stomach is an elastic j shaped hollow organ comprised of three major regions the superior and rounded region known as the fundus the central body and the inferior portion called the pylorus the stomach is also characterized by two curvatures a lesser and a greater curvature the stomach also contains two sphincters right the gastroesophageal sphincter and the pyloric sphincter now the cardiac region is the meeting point or the junction where the end of the esophagus connects to the beginning of the stomach as a result the gastroesophageal sphincter is also alternatively known as the cardiac sphincter this sphincter controls the opening into the stomach now the pyloric sphincter is a valve located at the junction between the pylorus of the stomach and the duodenum of the small intestine the presence of visible folds known as rugae in the mucosa layer of the stomach render the stomach elastic in other words the stomach is capable of undergoing stretch and expansion to store or hold food after food is digested the stomach returns to its original size or condition recall that the muscularis layer of the stomach contains an additional layer of smooth muscle known as the oblique muscle layer collectively three layers of smooth muscle render the stomach a very muscular organ and quite efficient at the process of churning churning is a type of mechanical digestion that mixes a food bolus together with hydrochloric acid to create a liquefied mixture known as chyme now chemical digestion is the process of breaking down large complex food molecules into smaller or simpler compounds via enzymatic action during chemical digestion there is the breaking or alteration of chemical bonds now hydrochloric acid found in gastric juice does not participate in chemical digestion of food only enzymes such as pepsin chemically digests food therefore churning which is the mixing of a food bolus together with hydrochloric acid is classified as mechanical digestion and not chemical digestion due to the acidic properties of hydrochloric acid this substance can corrode and perforate the stomach laying as a result the mucosa layer of the stomach secretes a protective coat of mucous to prevent any corrosion by hydrochloric acid this brings us to our next two checkpoint questions in a series you will see two questions back to back here's the first and here is the next now when unstretched or uncoiled the small intestine spans about 20 to 21 feet long the width or diameter of the small intestine ranges anywhere between three quarters of an inch to one inch the small intestine functions in the absorption of nutrients and the majority of the water found in ingested food and liquids the segments of the small intestine in sequential order are the initial duodenum the intermediate jejunum and the terminal ilium the longest segment of all three is the ilium now the innermost mucosal layer of the small intestine contains three absorptive structures known as the plique circularis villi and microbili visible macroscopic folds lining the mucosal layer of the small intestine are termed plique circularis or in other words circular folds present on each of these pleaking are microscopic finger-like projections called villi here is a magnified view of one single villus here we can see that the villi are lined with absorptive simple columnar epithelial cells note that each villus contains a network of blood capillaries and specialized lymphatic capillaries called lacteals absorbed nutrients are moved into the circulation by way of blood capillaries or lacteals here is a magnified view of a single absorptive simple columnar epithelial cell on the apical or free surface of the simple columnar epithelial cell are tightly packed microvilli brush border is the name assigned to the microvilli covered surface of simple columnar epithelial cells together all three structures the plinky circularis the villi as well as the microvilli increase the total surface area of the small intestine by about 600 times for maximized nutrient absorption these three structures give rise to a 600 fold increase in surface area versus a small intestine that was completely smooth and completely lacking in these three structures there are four enzymes secreted by the small intestine peptidase maltase sucrase and lactase now peptidase is an enzyme that hydrolyzes the peptide bonds linking [Music] amino acids this results in the release of free amino acids therefore we can establish that peptidase enzyme completes protein digestion the next three enzymes catalyze the breakdown of disatroids or double sugars into a monosaccharide known as glucose here maltase is an enzyme that hydrolyzes the disatroid maltose or malt sugar into glucose sucrase is an enzyme that hydrolyzes the disatroid sucrose or table sugar into glucose lastly lactase is an enzyme that hydrolyzes the disatroid lactose or milk sugar into glucose lastly lactase is an enzyme that hydrolyzes the disatroid lactose also known as milk sugar into glucose owing to its ability to hydrolyze and break down so many types of carbohydrates or sugars the small intestine is considered the primary site of carbohydrate digestion this brings us to our next checkpoint question here is the liver and accessory digestive organ located on the underside of the diaphragm in the upper right quadrant of the abdominal pelvic cavity owing to its secretory function the liver is also classified as a gland now the digestive function of the liver is to secrete bile the liver is bilobed so bile produced by each lobe of the liver is drained by a corresponding right or left hepatic duct the right and left hepatic ducts will merge to form the common hepatic duct bile produced by the liver will be conducted along the cystic duct towards the gallbladder where this bile is then concentrated and stored the gallbladder is a sac-like organ located underneath the liver now the common hepatic duct and the cystic duct merge to form the common bile duct which empties bile directly into the duodenum of the small intestine now the liver is required for our survival as it carries out numerous bodily processes in addition to its digestive function the liver stores vitamins and minerals and serves as a major detoxification organ breaking down harmful products such as alcohols and rendering them less harmful the liver also degrades old red blood cells to yield a yellow pigment known as bilirubin bilirubin is formed from the breakdown of hemoglobin found in red blood cells bilirubin is then secreted by the liver into bile to be eliminated from the body again the gallbladder is a small pear-shaped muscular storage sac that stores and concentrates bile produced by the liver the gallbladder holds bile produced by the liver until it is needed for digesting fatty foods in the duodenum of the small intestine upon the digestion of fatty foods the gallbladder will be triggered to squeeze its store of bile out of the cystic duct then along the common bile duct and eventually into the duodenum at the site of the duodenum is where the bile will encounter the fatty content in chyme and subsequently emulsify the fat molecules present bile has detergent properties thus interfering with the hydrophobic attractions between fat molecules that cause them to aggregate into large fat globules during the emulsification process bile will break up and disperse the large fat globules into smaller units called emulsified fat droplets since bile does not contain any enzymes there is no enzymatic breakdown of the chemical bonds in the fat molecules after emulsification by bile there is no chemical alteration of the fat molecules into a simpler compound after emulsification fat molecules still remain fat molecules therefore bile is considered a type of mechanical digestion not chemical digestion now the pancreas is an elongated flattened and feather-shaped digestive accessory organ as a gland the pancreas performs both exocrine and endocrine functions because the pancreas exhibits two different functions it is therefore classified as a heterochronic gland in other words the pancreas is both an exocrine and and endocrine gland now glands are classified as either exocrine or endocrine based on the site of product release exocrine glands release their products into a duct that leads to the external environment of the body the pancreas behaves as an exocrine gland because it secretes pancreatic digestive juice into the main pancreatic duct which then empties this pancreatic juice into the common bile duct and then eventually into the duodenum of the small intestine as part of the continuous digestive tract the duodenum is therefore opened or exposed to the external environment since the pancreas is not a tube-like organ that forms part of the continuous elementary canal the pancreas does not directly encounter food or kind the duodenum is where pancreatic juice encounters chyme directly now the pancreas also behaves as an endocrine gland because it secretes hormones directly into the blood note that groups of pancreatic cells called eyelets of langerhans secrete two antagonistic blood glucose regulating hormones glucagon and insulin the contents of pancreatic digestive juice include the enzymes amylase lipase and trypsin as well as a substance known as sodium bicarbonate amylase enzyme chemically digests carbohydrates lipase enzyme chemically digests triglycerides also known as fats and trypsin enzyme chemically digests proteins now trypsin continues chemical digestion of partially digested proteins by the stomach pancreatic juice is considered the most powerful digestive juice because of its ability to chemically break down all three macro nutrient groups carbohydrates fats and proteins now sodium bicarbonate is a substance that binds to and neutralizes the hydrochloric acid content in kind this is to prevent the hydrochloric acid from corroding the intestinal wall when unstretched or uncoiled the mucosa-lined large intestine spans approximately five feet long the width or diameter of the large intestine is about two and a half inches in comparison the large intestine is shorter in length but wider than the small intestine now the large intestine is an alimentary canal organ that forms the final section of the continuous digestive tract the large intestine is subdivided into the following sections the cecum which is a blind pouch that marks the beginning of the large intestine the ascending colon located on the right side of the abdominal pelvic cavity the transverse colon that extends horizontally across the abdominal pelvic cavity the descending colon located on the left side of the abdominal pelvic cavity the s-shaped sigmoid colon the rectum and finally terminating in the anus now recall that the small intestine absorbs the majority of the nutrition and water content in our ingested foods and liquids the large intestine will then absorb the remaining water and any remaining absorbable nutrients sending the indigestible material to the rectum this process converts liquid kind into semi-solid waste or fecal matter the large intestine does exhibit strong peristaltic waves to propel waste along during a diarrheal episode food waste moves through the large intestine too quickly for it to adequately absorb the remaining water as a result stool or fecal matter will contain a higher proportion of water on the other extreme constipation occurs when waste remains in the large intestine for too long as a result too much water is absorbed and the resulting stool or waste contains a very low proportion of water the cecum of the large intestine contains a worm like vermiform appendix this is an offshoot that hangs off the cecum and has no established digestive function the appendix is considered a vestigial or ancestral trait that has been greatly reduced from the original ancestral form and is no longer functional recall that the appendix is classified as mucosa-associated lymphoid tissue or malt therefore the appendix is capable of trapping bacteria which causes it to become obstructed and deeply inflamed this inflamed condition is known as acute appendicitis which must be surgically removed prior to its rupture this brings us to the first checkpoint question of this lecture recording in the large intestine the longitudinal layer of smooth muscle is reduced to a narrow band called the tania coley the tania coley is located outside of the ascending transverse descending and sigmoid colons now the ileo sequel sphincter or valve is a ring of muscle that connects the last segment of the small intestine known as the ilium to the first portion of the large intestine known as the cecum the ileocecal sphincter or valve functions to prevent the backflow or regurgitation of waste from the large intestine into the small intestine here the right colic flecture is used to describe the sharp bend or flexion point at the juncture of the ascending colon and the transverse colon the right colic flecture is also known as the hepatic flexure because the bend is located next to the liver recall that the liver is located in the upper right quadrant of the abdominal pelvic cavity the left colic flexure is used to describe the sharp bend or flexion point at the juncture of the transverse colon and the descending colon the left colic flexure is also known as the splenic flexure because the bend is located next to the spleen recall that the spleen is located in the upper left quadrant of the abdominal pelvic cavity now the external sphincter pictured here is under voluntary control whereas the internal anal sphincter not visible in this figure is under involuntary control this brings us to our next checkpoint question the peritonium is one of the double layered serous membranes of the body the peritoneum contains a visceral layer that directly covers the surface of digestive organs this layer is termed the visceral peritoneum the peritoneum also contains a parietal layer that lines the wall of the abdominal pelvic cavity this layer is termed the parietal peritoneum thus the visceral peritoneum is always deep or internal to the parietal peritoneum located between the visceral and parietal peritoneum is the peritoneal cavity which contains lubricating peritoneal fluid as the visceral peritoneum surrounding digestive organs glides against the parietal peritoneum lining the abdominal pelvic wall this creates friction the peritoneal fluid provides lubrication to reduce or minimize this friction here are cross-sectional and lateral views of the abdominal pelvic cavity to illustrate the visceral peritoneum the parietal peritoneum and the peritoneal cavity now a structure known as the mesentery is a double layer or fold of visceral peritoneum think of the mesentery as an extension of the visceral [Music] peritoneum the mesentery anchors the small intestine to the posterior abdominal wall this can also be seen in the lateral view of the abdominal pelvic cavity here we can see the mesentery anchoring the small intestine to the posterior abdominal wall the mesentery stabilizes the position of the small intestine and prevents its entanglement during digestive movements or abrupt changes in body position the mesentery also provides a pathway or conduit for nerves blood vessels lymphatic vessels and lymph nodes thereby allowing communication between the abdominal wall and the small intestine the omenta are also double layered sheets of visceral peritoneum but they are structurally thinner compared to the mesentery the visceral peritoneum covering the stomach extends on both sides into large double layered sheets filled with fat giving it a lace-like appearance the greater momentum arises from the greater curvature of the stomach and drapes over the front of the small intestines much like an apron then the greater omentum doubles back to ascend up to the transverse colon the lesser omentum extends from the lesser curvature of the stomach to the liver this is also visible along the lateral view where we can see the lesser omentum connecting the lesser curvature of the stomach to the liver the omenta function in fat deposition providing a storage area for adipose tissue they also act as a barrier by preventing the [Music] visceral peritoneum and parietal peritoneum from adhering to each other and creating friction omenta also help hold and maintain the abdominal organs in their proper positions here is a side-by-side comparison in the middle image we have a photograph of a cadavers apron-like or curtain-like greater omentum draping over the abdominal organs the greater omentum would need to be reflected as shown in the left image in order to expose the abdominal organs underneath now in the right image we have an illustration depicting the abdominal organs and the lesser omentum the lesser omentum is visible and exposed once the liver is reflected abdominal pelvic organs are classified as intraperitoneal if they are located inside or within the peritoneal cavity intraperitoneal organs include the stomach jejunum ilium cecum transverse colon sigmoid colon and the liver in the cross-sectional and lateral view we can see that these organs are located inside or within the peritoneal cavity in the cross-sectional view it is also evident that these intraperitoneal organs are completely covered on all sides by visceral peritoneum abdominal pelvic organs located behind the parietal peritoneum lining the abdominal pelvic wall are classified as retroperitoneal here retro translates to behind or back there are two categories of retroperitoneal primarily retroperitoneal and secondarily retroperitoneal primarily retroperitoneal organs originally develop behind the parietal peritoneum during embryonic development and continue to remain in this retroperitoneal position for the duration of our lifetime the only example of a primarily retroperitoneal organ is the kidneys which originated behind the parietal peritoneum during embryonic development and continues to remain in this retroperitoneal position for the remainder of our lifetime now secondarily retroperitoneal organs originally started out in the intra peritoneal position but then acquired their retroperitoneal position around the fifth week of embryonic development abdominal organs classified as secondarily retroperitoneal include the pancreas duodenum ascending colon descending colon rectum and urinary bladder around the fifth week of embryonic development the growth of other organs most notably the small intestine forces all secondarily retroperitoneal organs backwards as these organs are displaced backwards the mesentery of these organs become resorbed and lost as a result their visceral peritoneum fuses with the parietal peritoneum lining the abdominal pelvic wall now these organs end up in the retroperitoneal position and only their anterior surface is covered by parietal peritoneum in the upper left image we can see the mesentery in the process of disintegrating and being resorbed as organs such as the pancreas and duodenum are pushed backwards and end up behind the parietal peritoneum in the retroperitoneal position in the bottom figure we can see the parietal peritoneum denoted by the green outline along this lateral or midsagittal view it's also evident that the pancreas duodenum rectum and urinary bladder are located behind the parietal peritoneum hence their appropriate classification as retroperitoneal organs along this same view we can see the liver stomach transverse colon jejunum and ilium located inside or within the peritoneal cavity hence their appropriate classification as intraperitoneal organs this brings us to our next checkpoint question mechanical digestion physically breaks down food but does not chemically alter the structure of molecules examples of mechanical digestion include chewing of food in the mouth or oral cavity propulsion of food via involuntary peristaltic waves mixing of a food bolus with hydrochloric acid in the stomach mixing of chyme with intestinal juices during segmentation and lastly emulsification of fats by bile these are all examples of mechanical digestion now chemical digestion involves hydrolysis which is the enzymatic breakdown of food or nutrient molecules in other words chemical digestion involves the actual breaking of chemical bonds by enzymes chemical digestion does alter the resulting structure of food molecules this brings us to a summary of chemical digestion involving carbohydrates proteins and fats recall that the primary site of carbohydrate digestion occurs in the small intestine now salivary amylase chemically digests polysaturates such as starch into the disatrod maltose the same enzyme is also secreted by the pancreas so any starch that manages to reach the duodenum undigested will still be chemically broken down by pancreatic amylase in other words any starch that manages to escape the oral cavity undigested by salivary amylase will eventually be broken down by pancreatic amylase in the small intestine intestinal enzymes digest disatrods into monosaccharides now the intestinal enzyme maltase chemically digests the disatrod maltose into the monosaturated glucose the intestinal enzyme sucrase chemically digests the disatrod sucrose into the monosaccharide glucose and fructose lastly the intestinal enzyme lactase chemically digests the disatroid lactose into the monosaccharides glucose and galactose in summary the final absorbable breakdown products of carbohydrate digestion are the monosaccharides glucose fructose and galactose chemical digestion of large polypeptides called proteins first begins in the stomach courtesy of the gastric enzyme known as pepsin pepsin partially digests large polypeptides into small polypeptides at the site of the duodenum is where the pancreatic enzyme trypsin continues digestion of those small polypeptides into peptides now a peptide can contain anywhere between 2 to 50 amino acids lastly intestinal enzyme peptidase will chemically digest peptides into individual amino acids thus protein digestion is completed by peptidase in the small intestine in summary the final absorbable breakdown product of protein digestion is amino acids while bile does not chemically digest fats bile does emulsify fat globules into smaller fat droplets smaller fat droplets have greater surface area this in turn provides pancreatic enzyme lipase with greater access to the fat molecules lipase can now more effectively chemically digest fat into fatty acids monoglycerides and glycerol in summary the final absorbable breakdown product of fat digestion are fatty acids monoglycerides and glycerol now absorption is the movement of small nutrient subunits such as glucose fructose galactose amino acids fatty acids monoglycerides and glycerol from the intestinal tract into blood or lymph recall that the majority of nutrient and water from ingested food and liquids is absorbed in the small intestine the large intestine absorbs any remaining water and nutrient content still left over here is a table outlining the enzymes secreted by each digestive organ the nutrient molecules they chemically digest or hydrolyze and the resulting breakdown product or subunit this brings us to our final checkpoint question of this lecture recording